This invention relates to a method and apparatus for freezing products and, more particularly but not exclusively, is concerned with a method and apparatus for freezing foodstuffs.
The use of liquid nitrogen to freeze foodstuffs has increased dramatically over the past 30 years. The improvement in the quality of the frozen food is well known. However, whilst liquid nitrogen is now used for freezing premium food products its cost prevents it being used for freezing those foodstuffs which do not command a premium price. These foodstuffs are typically frozen using mechanical refrigeration.
Over the years many attempts have been made to reduce the quantity of liquid nitrogen required to freeze a given foodstuff and gradually it has become economically viable to use liquid nitrogen to freeze an increasing range of foodstuffs.
The present invention aims to continue this trend.
According to the present invention there is provided a method of freezing a product, which method comprises the steps of vaporising a cryogenic liquid and warming the vapour thus formed in indirect heat exchange with a product to be frozen, work expanding the warmed vapour, and using the work expanded vapour thus obtained to refrigerate the or another product.
Whilst the cryogenic liquid will usually be liquid nitrogen, it could also comprise another cryogenic liquid, for example liquid air.
If desired, the work expanded vapour may be brought into direct heat exchange with said product to refrigerate the same. Alternatively, the work expanded vapour may be brought into indirect heat exchange with said product to refrigerate the same.
Advantageously, said method includes the step of using the work recovered during said work expansion to heat water.
Alternatively, or in addition, said method may include the step of using the energy recovered during said work expansion to drive a turbulence inducing fan.
Alternatively or in addition, said method may include the step of using at least part of the work recovered during said work expansion to at least partially power a mechanical refrigerator having a refrigerated space.
In one embodiment, said method includes the step of passing said product through said refrigerated space after freezing it with cryogenic fluid.
In another embodiment, said method includes the step of passing said product through said refrigerated space before freezing it with cryogenic fluid.
Advantageously, said cryogenic liquid is liquid nitrogen and said method includes the step of supplying liquid nitrogen at a pressure greater than 5 bar g, preferably greater than 10 bar g, more preferably greater than 15 bar g, and advantageously less than 20 bar g.
Preferably, said product is a foodstuff.
The present invention also provides an apparatus for freezing a product, which apparatus comprises a freezer, a heat exchanger in said freezer, and a work expander, the arrangement being such that, in use, cryogenic liquid can be vaporised and warmed in said heat exchanger, the vapour thus formed expanded in said work expander, and then used to further refrigerate product in said freezer.
Advantageously, said apparatus further comprises a second heat exchanger for conveying expanded vapour from said work expander through said freezer in indirect heat exchange with said product.
Preferably, said apparatus includes means to transfer energy from said work expander to water.
Advantageously, said work expander is connected to a fan for inducing turbulence in said freezer.
Preferably, said apparatus includes a mechanical refrigerator having a compressor associated therewith, and means for, in use, transferring energy from said work expander to said compressor.
In one embodiment, said work expander may be directly coupled to said compressor.
In another embodiment said work expander is connected to a generator, said compressor is connected to a motor, and said generator is connected to said motor.
Preferably, said apparatus includes a power control unit, wherein said generator is connected to said motor via said power control unit.
Advantageously, said power control unit is connectable to mains power and is capable, in use, of directing energy from said mains power to said motor as required.
Preferably, said mechanical refrigerator includes a heat exchanger arranged to cool compressed refrigerant from said compressor in heat exchange with said expanded vapour from said freezer.
Advantageously, said mechanical refrigerator comprises a refrigerated space.
In one embodiment, said refrigerated space is disposed downstream of said freezer.
In another embodiment, said refrigerated space is disposed upstream of said freezer.
In a further embodiment there are two refrigerated spaces (which may be associated with a single mechanical refrigerator or separate and distinct mechanical refrigerators) one of which is disposed upstream of said freezer and the other of which is disposed downstream thereof.
Preferably, said apparatus further comprises a pump to raise the pressure of said cryogenic liquid prior to entering said heat exchanger.
Advantageously, said pump is capable of delivering liquid nitrogen at a pressure of at least 10 bar g.
For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: